Insight into the Atomic Structure of High-Voltage Spinel LiNi_0.5Mn_1.5O₄ Cathode Material in the First Cycle

Abstract

Application of high-voltage spinel LiNi 0.5 Mn 1.5 O 4 cathode material is the closest and the most realistic approach to meeting the midterm goal of lithium-ion batteries for electric vehicles (EVs) and plug-in hybrid electric vehicles (HEVs). However, this application has been hampered by long-standing issues, such as capacity degradation and poor first-cycle Coulombic efficiency of LiNi 0.5 Mn 1.5 O 4 cathode material. Although it is well-known that the structure of LiNi 0.5 Mn 1.5 O 4 into which Li ions are reversibly intercalated plays a critical role in the above issues, performance degradation related to structural changes, particularly in the first cycle, are not fully understood. Here, we report detailed investigations of local atomic-level and average structure of LiNi 0.5 Mn 1.5 O 4 during first cycle (3.5−4.9 V) at room temperature. We observed two types of local atomic-level migration of transition metals (TM) ions in the cathode of a well-prepared LiNi 0.5 Mn 1.5 O 4 //Li half-cell during first charge via an aberration-corrected scanning transmission electron microscopy (STEM). Surface regions (∼2 nm) of the cycled LiNi 0.5 Mn 1.5 O 4 particles show migration of TM ions into tetrahedral Li sites to form a Mn 3 O 4 -like structure. However, subsurface regions of the cycled particles exhibit migration of TM ions into empty octahedral sites to form a rocksalt-like structure. The migration of these TM ions are closely related to dissolution of Ni/Mn ions and building-up of charge transfer impedance, which contribute significantly to the capacity degradation and the poor first-cycle Coulombic efficiency of spinel LiNi 0.5 Mn 1.5 O 4 cathode material. Accordingly, we provide suggestions of effective stabilization of LiNi 0.5 Mn 1.5 O 4 structure to obtain better electrochemical performance.

Density functional theory analysis of surface structures of spinel LiNi0.5Mn1.5O4 cathode materials

“…Defect formation energies and diffusion barriers of Ni and Mn are much lower in the (110) surface than those in the bulk material. TM tetral -Vac TM complex could be easily formed on (110) surfaces, which strongly supports the previous experimental observation [27]. …”

“…The migration of the Ni/Mn ions from the octahedral sits to the tetrahedral sites has also been observed in the spinel LNMO upon heating [9,27,35,36].…”

“…The (100) and (111) STEM observation showed that the delithiated LNMO samples have different atomic configurations compared with the spinel structures at the surface upon initial delithiation [27].…”

“…They found that Ni and Co show strong facet selectivity when building up their surface segregation layers: Ni prefer to segregate on (200) crystalline plane of Li-Mn-rich oxide whereas Co has a strong preference to (20-2) one [26]. Lin et al [27] found using an STEM that a layer of Mn 3 O 4 -like structure with a thickness of ~2 nm was observed on the surface of the LNMO cathode materials during the initial charging to 4.9 V. They found that the surface phase transition resulted in the diffusion of Ni/Mn ions into the Li ion positions. DFT simulations have been successfully used to show that the O deficiency will promote Ni/Mn ions diffusion from octahedral positions to tetrahedral Li ion positions in the bulk LNMO [9].…”

Direct Experimental Probe of the Ni(II)/Ni(III)/Ni(IV) Redox Evolution in LiNi0.5Mn1.5O4 Electrodes

“…Recently, surface rearrangement with inactive Ni 2+ phase has been reported [31][32] . Moreover, reactions between the LiNi0.5Mn1.5O4 electrodes and electrolytes lead to the deposition of a decomposition product of the electrolyte and the reduction of transition metals in the active materials 30 .…”